This invention relates to a lens assembly and a method for its preparation, and more particularly to such a lens assembly that is stable and is accomplished in a relatively low volume.
Optical elements such as lenses are assembled together to form an optical system. In one common form of the optical system, the individual lenses are held in a fixed relation to each other to form a lens assembly with a ray path through the lens assembly. As a result, the focal plane of the optical system is relatively stationary.
The lenses are typically fixed in position inside a lens housing. The lens housing serves as the structural element that holds the lenses. The lens housing also protects the lenses and prevents stray light from entering the optical system. However, the lens housing also increases the radial size of the optical system, which is undesirable in some circumstances where the optical system must fit within a tightly confined space.
The conventional approaches to fixing the lenses inside the lens housing include adhesive bonding of the lenses to the inner side wall of the lens housing and mechanical screw fittings that contact the lenses and hold them in position. Adhesive bonding has the advantage that the radial size of the lens housing need not be increased to accommodate the bonding, but it has the disadvantage that the adhesive may deform over time and apply stress to the bonded lenses that causes them to deform. Additionally, the available adhesives deform as a result of exposure to humidity and temperature changes, so that the relative position of the lenses changes and the ray path and focal plane are altered. Adhesive bonding is also difficult to perform for long lens assemblies that require a long, relatively small-diameter lens housing. Mechanical screw fittings require that the radial size of the lens housing be increased to provide for the male and female threads on the mating elements, which may render the optical system unusable for some applications. Mechanical screw fittings are also relatively expensive to produce. Other, more complex arrangements may be envisioned for specialized applications, but these complex arrangements are too costly for many applications.
There is a need for an approach to lens assemblies that holds the lenses in a well-defined, fixed relation in a lens housing, but does not require increasing the radial size of the housing. The present invention fulfills this need, and further provides related advantages.
The present invention provides a lens assembly and a method for its preparation. The lens assembly achieves the advantages of using a protective lens housing, but does not place undesirably large stresses on the lenses. Temperature, humidity, and other environmental effects on the optical performance of the lens assembly are minimized. The radial size of the housing is not increased over the minimum size required for the lenses and the wall thickness of the housing. The present approach is also relatively inexpensive to implement.
In accordance with the invention, a lens assembly comprises a lens housing having an inner side wall and hollow interior, wherein the lens housing is made of a lens-housing material; a first lens received in the hollow interior, wherein the first lens is not affixed to the inner side wall; and a second lens received in the hollow interior and axially spaced from the first lens, wherein the second lens is not affixed to the inner side wall. An annular first retainer is within the hollow interior and is disposed between and contacting the first lens and the second lens, wherein the first retainer is made of a first-retainer material. A first-retainer joint affixes the first retainer to the inner side wall of the housing. The first-retainer joint is made of a first-retainer joint material selected from the group consisting of the lens-housing material, the first-retainer material, and a solder. In one form, the inner side wall is cylindrical in shape, and the lenses and the first retainer are substantially cylindrically symmetric.
In one embodiment, the inner side wall of the housing includes a seat. The first lens has a first side and a second side. The first side of the first lens rests against the seat, and the first retainer contacts the second side of the first lens. The seat may be in the form of a step in the inner side wall, or another annular retainer affixed to the inner side wall.
The first-retainer joint material may be metallic or nonmetallic, but it is preferably comprises a metallic material that is stable under temperature changes and environmental effects such as humidity. The metallic form of the first-retainer joint material may be melted metal of the first retainer material, with the first-retainer joint formed by reflow of the material due to general or local heating. It may instead be melted lens-housing material or solder. In any case, where the first lens is contoured, the annular first retainer may be conformably contoured to a contacted portion of the first lens.
The principles of the present approach may be extended to the use of additional retainers and additional lenses. Thus, in one embodiment the second lens has a first side and a second side, the first retainer contacts the first side of the first lens, and the lens assembly further includes an annular second retainer contacting the second side of the second lens. A second-retainer joint affixes the second retainer to the inner side wall of the housing. Features discussed elsewhere herein may be utilized in relation to such extended forms.
A method for preparing a lens assembly comprises the steps of providing a lens housing having an inner side wall and a hollow interior, providing a first lens having a first side and a second side, placing the first lens into the interior of the lens housing, providing an annular first retainer, placing the annular first retainer into the interior of the lens housing contacting the second side of the first lens, and affixing the annular first retainer to the inner side wall with a first-retainer joint by an approach selected from the group consisting of soldering and welding. The first lens is not affixed to the inner side wall.
In the conventional approach of using an adhesive to affix the lenses to the inner side wall, the radial stresses resulting from the adhesive bonding may deform the lenses, particularly where the lenses are made of plastic rather than glass. In the present approach, the lenses are not themselves affixed to the inner side wall, but instead are placed into the lens housing alternating with the retainers. The retainers are affixed to the inner side wall, so that there is no stress loading directly onto the lenses. Any stresses in the lenses resulting from the retainers contacting the lenses are largely in the through-thickness direction (i.e., parallel to the optical axis) of the lenses at their radial peripheries, so that there is minimal deforming of the lenses either at the assembly temperature, or when the temperature is later changed.